In order to ensure safety of passengers in a automobile at the time of collision, improvement of the collision safety property of a automobile body is increasingly on demand in recent years. Due to this, in the automobile body, increasing of the strength of the side portion of a automobile i.e., increasing of the strength of a automobile door is particularly required and thus a bar for reinforcing a automobile door is always provided in a automobile door, in recent years. Here, in order to reduce the weight of a automobile body, a steel tube is increasingly in use for the bar for reinforcing a door.
A steel tube for a automobile door reinforcing bar is required to have high strength, so that the automobile door reinforcing bar can achieve the intended effect in application thereof. Therefore, a steel tube whose strength has been increased is generally used for a automobile door reinforcing bar. Conventionally, a electric resistance welded tube is used as a steel tube for automobile door reinforcing bar. Specifically, the off-line QT (quench and temper) type steel tube whose strength has been increased by the off-line QT treatment such as induction quenching has conventionally been used, or the as rolled type steel tube which is produced by electric resistance welding a steel sheet having high strength has conventionally been used (here, the steel sheet is strengthed by the QT treatment at the stage of producing a thin steel sheet as the base material of a electric resistance welded tube).